This disclosure is related to the technology of reducing the sludge (precipitation) in the process of wet etching and thinning of glass using hydrofluoric acid (HF) and etchant containing HF. The existence of sludge in etchant significantly interrupts the etching process and reduces the quality of etching product. Sludge (or precipitation) in the etchant (or etching bath) induces defects such as pimples and rimples (wrinkles) to a glass surface during etching. In addition, the accumulation of sludge in the etching bath can severely hinder the glass etching operation and it also results in deposition of by-products on the surface of the etched glass which requires brush cleaning to meet the stringent optical and cosmetic requirements. Contact cleaning such as brush cleaning can damage a fresh glass surface and compromise the mechanical performance of the glass. The prevention or reduction of the precipitation in etching bath or etchant improves the efficiency of glass etching process, and also reduces costs regarding material consumption and machine maintenance.
The methods of the addressing sludge issues can be divided into two categories: physical separation and chemical inhibition. The method of physical separation focuses on separating the sludge (solid) from an etchant such as HF (liquid), which is a physical process to regenerate a sludge free etchant. Methods of filtering used HF solutions containing sludge. are disclosed in JP-04370737B2, JP-2003012305A, CN-1393418A, and TW-527320B. The sludge can be removed by filtration and centrifugation. In patents KR-943321B1 and KR-2009094578A, HF and sludge are separated by heating the solution containing HF and sludge. In this way, HF is recycled by collecting HF from the gas phase, and sludge is condensed by heating. However, the efficiency of separation can be low, and the cost for operation is also relatively high for this method. Overall, while physical separation can recycle the active component (for example HF) from the used etchant, and separate the sludge of large particulate dimensions. there are limitations to the physical method including:                (1) There is a requirement of sludge separation equipment, which increases the cost for equipment purchasing and maintaining.        (2) The use of a separation method such as heating increases the energy consumption.        (3) The separation can only partially remove sludge from etchant. For example, while a sludge of large size particles may be easily removed by centrifugation or filtration, small sludge particles stay in etchant and continuously impact the performance of the recycled etchant.        (4) The formation of sludge is a slow process, which means sludge will form even after physical separation.Thus, the sludge problem cannot be completely resolved by employing a physical method alone.        
The chemical method is complementary to the physical method for sludge management. The chemical method reduces or inhibits the formation of sludge in the etchant rather than separating the sludge from etchant. Therefore, the chemical method possesses several advantages which include:                (1) It does not require the installation of experiment equipment.        (2) The process does not consume extra energy.        (3) The process can inhibit the formation of sludge or reduce the quantity of sludge.        (4) The cost for glass etching can be reduced by applying an appropriate chemical strategy for etching.Combining the chemical method together with the physical method, the sludge problem can be managed more effectively and efficiently.        
Currently, there are few references that report the chemical method for sludge management. Patents KR-2011032432A and KR-2011056095A disclose a strategy of stabilizing the sludge particles in solution and preventing them from aggregation by adding surfactant molecules into etchant. This method reduces the adsorption of the sludge particles on glass, and reduces the formation of defects during etching. However, sludge and sludge particles still exist even after the addition of surfactant. Further, the quantity of sludge does not change. These sludge particles can still induce defects and cause problems as the quantity of sludge increases. The method does not fundamentally resolve the problem of sludge.
Overall, the chemical method for sludge management is promising because of several advantages: (1) Effective in sludge reduction, (2) Low cost, and (3) Ease of implementation. However, there are several challenges which include:                (1) Determining a systematic, chemical strategy for controlling the sludge formation is still lacking.        (2) The correlation of etching chemical factors on sludge manipulation have not been claimed or published.        (3) A fundamental understanding of sludge formation is still unknown.        (4) The chemical (only) strategy for sludge reduction has only been developed for limited types of glasses such as ion-exchanged Corning® Code 2318 an alkali alumino silicate glass. However, the principles involved are applicable to other types of glass, for example, Corning® Eagle XG™ which is an alkaline earth aluminoborosilicate glass.        
The present disclosure provides a solution to the forgoing challenges, and describes a systematic chemical strategy based on a an understanding of sludge formation and the correlation of chemical and environmental factors.